1. Field of the Invention
The present invention relates to a cryocooler system and a superconducting magnet apparatus having the same.
2. Description of the Related Art
In the prior art, a magnetic resonance imaging (MRI) apparatus uses a superconducting magnet. Because a superconducting magnet maintains a superconducting state at an ultra-low temperature such as 4.2 K, a cooling system for maintaining the ultra-low temperature is required.
A two-stage cryocooler that is conventionally employed in a cooling system in the prior art includes a first stage unit that operates at a temperature, for example, in a range from 40 K to 50 K and a second stage unit that operates at a temperature, for example, of 4 K. The second stage unit cools a superconducting body by directly and indirectly thermally contacting the superconducting body, for example, a superconducting coil, and the first stage unit cools a heat shielding unit that reduces heat transfer between room temperature and the superconducting body.
In a cooling system of an MRI apparatus of the prior art, heat is generated from a resistance of current leads when a superconducting magnet is ramped-up or ramped-down to generate or to turn off the resulting magnetic field. Accordingly, a cooling system needs to provide against the heat generated from the current leads. In the prior art, a bath cooling system has been used for cooling the superconducting magnet. In the bath cooling system, the heat generated when the superconducting magnet is ramped-up or ramped-down is cooled by placing the current leads in a flow of helium vapor that is vaporized from a liquid helium supplier. However, the bath cooling system uses an excessive amount of helium. In the case of a cooling system such as a thermosiphon method or a cryogen-free method, the cooling of the helium vapor is not useful and the heat generated when the superconducting magnet is ramped-up or ramped-down may not be sufficiently cooled due to the limited cooling capability of a cooler.